This invention relates to a steering system for an articulated vehicle that operates by hydraulic steering cylinders or jacks, and more particularly, to a two-part steering system, including a pair of wagon-steer cylinders connected to the front axle (or rear axle) mounted in a wagon-steer mode for effecting relatively small directional changes of the steering at high speeds and sequentially operating therewith, a conventional pair of articulated steer cylinders for effecting the larger directional changes of the steering desired for maneuvering at low speeds.
Certain off-road earth moving vehicles, such as tractors, loaders and scrapers and log skidders, as well as articulated agricultural tractors, are effectively steered through hydraulic controls rather than by means of direct mechanical linkages. Articulated vehicles of this type usually have two body sections with a pivot therebetween and a pair of hydraulic cylinders, connected between the two body sections, with one cylinder mounted on each side of the pivot connection and hydraulically connected to work together. The steering is accomplished by extending one cylinder, while simultaneously retracting the other, to force pivoting of the vehicle parts at the joint relative to each other. Conventionally, an axle supports each vehicle part, and steering is accomplished solely through articulation of the vehicle while full power is delivered to both axles.
To control the cylinders in an articulated steering system, a steering wheel is utilized which operates the conventional pilot valves to pressurize a hydraulic pilot operated steering control valve connected between the steering cylinders and a source of pressurized hydraulic fluid flow for operation of the steering system. Accordingly, rotation of the steering wheel moves the above valve controls and actuates the steering cylinders to turn the vehicle in a first direction, and rotation of the wheel in the opposite direction moves the valve controls to produce a reverse operation of the steering cylinders. The hydraulic circuit includes a conventional hydraulic operated feedback system which senses the pivoting movement and deactivates the steering cylinders when the turning of the steering wheel ceases so that the operator may steer the vehicle by manipulation of the steering wheel in a conventional manner similar to that shown in U.S. Pat. No. 3,444,948.
An advantage of articulated-steer vehicles is the direct transmission connection to its solid axles which eliminates driving power loss due to universal drive joints found in other types of vehicles. However, basic requirements of acceptable steering system for an articulated steer vehicle is a directional control response that gives good low speed maneuverability and adequate high speed handling. Typically, the steering system of an articulated-steer vehicle exhibits a directional control response that is proper for low speed maneuverability, but limits the top speed capability of the vehicle as compared to Ackerman-steer vehicles.